The present invention is directed toward improving the adhesion and adhesion retention between a rubber composition and metallic reinforcement cord, such as steel wire and cable which is embedded in the rubber stock. Flat sheets or strips of such stocks, reinforced with metal or fibers, are utilized as plies or other components in the manufacture of tires, repair stocks for retreading tires, conveyor belts, hoses and the like and are referred to in the art as rubber skim stocks. Skim refers to a relatively thin layer or coating of the rubber over the reinforcement filaments or cords. Greater thicknesses of rubber can also be bonded to metal in other instances such as motor mounts and golf club shafts.
In the manufacture of the foregoing rubber articles, particularly steel-belted bias and radial tires, it has become common to reinforce the rubber skim stock material with steel wire or cable. One of the more important uses for a metallic reinforced rubber is as a belt where one or more of these belts are substantially circumferentially oriented beneath the tread stock to maintain the integrity and shape of the tire during inflation and subsequent load. Other areas where metal reinforced rubber skim stock may be utilized is in the body ply, bead or chafer of a tire.
There are known methods for promoting adhesion between vulcanizable rubber and steel reinforcement cords. A variety of metallic salts or complexes or other additives have been employed as coatings to the metal or as an ingredient in a rubber composition. For example, steel reinforcement cords are commonly plated with metals, such as brass, zinc or bronze, that are designed to promote and maintain adhesion to sulfur-vulcanized rubber. It is also common to incorporate adhesion promoters into the rubber compounds themselves. For example, such adhesion promoters can include cobalt salt additives, HRH systems (hexamethylene tetramine, resorcinol and hydrated silica), silanes, and the like. In particular, the adhesion of wire to rubber skim stock has been accomplished for years in the tire industry by the use of brass plated steel wire, and a specially formulated rubber containing high sulfur, resin and cobalt salt. However, incorporation of adhesion promoters into the rubber or as a coating for wire, can modify the working properties and performance of the vulcanized compositions, in particular the resistance to thermal and thermo-oxidizing aging. Furthermore, the incorporating of these compounds in the compositions is costly and the metals of these compounds may at times be scarce. In these systems, the adherence performance of the reinforcements obtained are at times insufficient and the bonding obtained can exhibit degradation over the course of time, poor resistance to thermal aging and/or thermo-oxidizing aging, in particular corrosion in the presence of water.
In a continuing effort to improve both the initial and, most importantly, the aged adhesion, alkoxy organosiloxane chemistry has been shown to be promising. Recently, we have discovered that incorporation of a compound comprising an aminosilane, a mercaptosilane, or a mixture of these, into the rubber composition prior to curing, improves metal adhesion and metal adhesion retention properties between a vulcanizable rubber composition and plated or unplated (e.g., bright) steel, and also improves thermal and humidity ageing. (See, for example, U.S. Pat. No. 7,201,944.)
However, other techniques, such as applying a mixture of alkoxy organosiloxanes in an alcoholic solution to metal plated wire cords, followed by heat setting of the film, is problematic in that the adhesive solution has a limited lifetime with respect to moisture curing. The film itself is insoluble in water and environmental problems are caused by the evolution of the alcohol solvent as a volatile organic compound (VOC).
Therefore, there is still a need to provide a method of treating wire to give a coating that will promote adhesion of a rubber stock to the wire during cure. There is also a need to provide such an adhesive coating to wire that is either unplated or metal plated. Further, there is also a need to provide an adhesion promoter that can be used with all types of rubber and does not require the use of special adhesive additives to the rubber vulcanizates, such as, but not limited to, cobalt, resins and high sulfur levels. In particular, there is a need to improve the adherence performance of the reinforcements to obtain sufficient bonding that is resistant to degradation over the course of time, especially resistance to thermal aging and/or thermo-oxidizing aging, in particular corrosion in the presence of water.